Dental articulators for simulating mandibular joint movements usually consists of a lower member for accommodating a cast of the mandible or maxilla, generally made of plaster, and an upper member for accommodating the corresponding other cast. The lower member and upper member are provided with magnetic holders or other mounts.
In this context, the connection between the upper and lower members is made by condylar joints, which permit movement corresponding to the human mandibular joint. For this purpose, the rear end of the lower member is provided with two vertically arranged posts some distance apart, on which condylar balls are located, these engaging variable-angle condylar housings with corresponding guide elements from below. Depending on the design, it is, of course, also possible to locate the condylar housing on the posts and, accordingly, the condylar balls on the upper member, as described in U.S. Pat. No. 4,981,437, for example.
Furthermore, the front area of the lower member is provided with an incisal pin, on which the upper member rests via an incisal guide table. The incisal pin is used for anterior guidance when simulating the protrusion, retrusion and laterotrusion movements. Posterior guidance when simulating the transverse and sagittal movements on the mandibular joint side is provided by the guide elements, which are provided with sagittal condylar guides and transverse Bennett guides.
In order to be able to simulate these movements, the condylar balls are capable of movement in the three translatory and rotatory degrees of freedom within the condylar housings, or the condylar housings about the condylar balls. In addition, the angles of the condylar housings can also be varied.
The disadvantage of these dental articulators is the complicated structure of the condylar housings and the need to use a host of different guide elements, occasionally with three-dimensional guides or guides with complex curvatures, in order to reflect the natural situation. This results in the additional difficulty that the joint mechanism of the dental articulator first has to be adjusted prior to use. This is usually done by adjusting the position in space of the condylar ball in relation to the joint axis running through the condylar joint. Owing to the curvature of the sagittal condylar guide and the Bennett guide, it is impossible to achieve statically determined allocation of the condylar ball within the condylar joint. For this reason, the Bennett guides have to be replaced by straight guides for adjusting. These adjusting guides enclose an angle in relation to each other, at which the condylar ball is pressed and retained during adjustment.
Another disadvantage of this prior art is that the condylar housing sitting on the condylar ball is only accessible from below. Consequently, the upper member of the dental articulator has to be removed in order to change inserts. Moreover, the movement of the condylar ball in the condylar housing cannot be observed.
However, for purposes of functional diagnostics, another movement is of interest, which can only be reproduced incompletely, if at all, in articulators of the familiar design.
When a patient closes his lower jaw about the terminal hinged axis, it is often the case that, when contact is made, only one antagonistic contact occurs on only one antagonistic pair of teeth. As the jaw is closed further, the lower jaw is subjected to the automatic influence of the teeth and slides into a position with the best possible, stable tooth contact. This is the intercuspital position. The mandibular joints are dislocated accordingly in this context.
The direction and the extent of this three-dimensional displacement of the mandibular joints under the influence of the teeth are of importance.
As the condylar housings of conventionally designed articulators prevent movement of the condyles in an upward, rearward and possibly a lateral direction, these articulators do not permit the simulation and measurement of this position. For this purpose, plaster casts are therefore commonly transferred to special "joint position measuring instruments", the design of which is similar to that of articulators, but which are not restricted by condylar mechanisms. The condylar housings are replaced by corresponding measuring devices in this case.